Even though its roots go back to 4,000 BC, metal casting remains one of the most popular processes for producing metal parts today. Increasingly, foundries are adopting additive manufacturing (AM) to support metal casting and improve production efficiency by creating patterns and moulds faster and more accurately than using conventional techniques. Here Joe Godfrey, 3D print specialist at Tri-Tech 3D, UK provider of Stratasys 3D printing solutions, explores the role of AM in foundries.
According to Casting the Future, an organisation that champions the casting industry, there are around 400 foundries in the UK. These range from die casting companies and investment foundries to sand foundries — each producing parts for industries including automotive, rail, power generation, aerospace, medical and marine. Many businesses are working hard to stay competitive in a sector heavily affected by the pandemic, supply chain disruptions and higher energy prices.
3D printing can support foundry productivity by efficiently producing moulds and patterns with complex geometries. In my experience, while many US foundries have already adopted 3D printing, uptake is lower among UK and European sites. This means many UK sites are yet to fully appreciate AM’s productivity value and how it complements traditional casting processes.
The trouble with tradition
Metal casting is expensive, with much of this cost linked to the production of moulds and patterns. For example, sand moulds require the fabrication of a master pattern — sometimes by manual sculpting — which the expendable mould forms around. Non-expendable (reusable) moulds often use tool steel or aluminium, which is expensive and can only be cut into a master-pattern using high-end machining equipment.
Another issue is lead times — mould production and delivery for investment casting can take weeks, even months. Once the mould has arrived, it can be several days before the foundry injects wax to create the patterns and casts all the required parts. These long lead times can have serious consequences in fast-moving industries and create unacceptable delays elsewhere — causing customer frustration.
To help overcome these costs and delays, moulders and pattern makers are introducing 3D printing into their workflows. This technology can help them eliminate time-consuming, conventional pattern and mould production while allowing significant design freedom.
Investment casting
Increasingly, pattern makers in investment casting foundries are using stereolithography (SLA) systems to print the patterns they traditionally created with wax. Often, pattern makers have limited design freedom when using wax, and this material is not always suitable for crafting more complex parts. For example, a large turbine rotor has several intricate features, and, using AM, foundries can print them in sections before combining them to create a single pattern ready for casting. Also, if a manufacturer only needed, say, 50 parts, it wouldn’t be economically viable to fabricate a mould. Instead, they could 3D print the patterns at a low volume in just a few days and start casting in a week.
Traditional investment casting involves a careful dewaxing process, where the pattern is melted away from the ceramic shell before molten metal is poured in. AM, on the other hand, produces resin-based patterns, which foundries must burn away. For the process to be a success, the pattern must burn out cleanly, have a lightweight build and offer good green strength so that foundry workers can handle the pattern and remove any support structures without distorting the pattern.
For titanium and other exotic metals, it is imperative that there is no contamination of the final metal casting — especially for aerospace parts. Most existing stereolithography resins contain antimony in their formulation — the ash of which could contaminate the final casting. Stratasys now supply Somos® WaterShed AF, which is antimony-free and burns out with very little ash (typically less than 0.005 per cent by volume). This makes it an ideal resin for producing defect-free castings.
Foundries can also use AM systems to print moulds, which are usually made from high-grade alloys like aluminium. Using a high temperature-resistant and stiff SLA resin, such as Somos® PerFORM, manufacturers can create moulds overnight that are ready to inject with wax the next day. While this additive approach may not always be practical for high-volume mould production, it can help foundries test the suitability of different geometries for small-scale outputs.
Sand casting
Usually, tooling costs for sand casting are minimal because an aluminium mould isn’t used. Even so, there’s always room for improvement, and 3D printing can help by optimising patternmaking. Whether a foundry uses green or air set sand casting, it will typically involve patterns made from wood, plastic or metal. Again, this can lead to several weeks of delays while a bespoke version is specified, ordered, manufactured and delivered to the foundry. Instead, by using an AM system, foundries can create a digital CAD design, print a pattern on site, and start sand casting in just a few days.
Whichever sand casting process is used — green or air set — it’s important that foundries select a strong 3D printing resin that can withstand being pressed into the sand. For example, some sand casting facilities use the Somos® WaterShed XC 11122 SLA resin because of its durability and ABS- and PBT-like properties. It also absorbs very little moisture and can remain dimensionally accurate over long periods. This is important if the time between pattern production and casting is long — especially in humid environments.
Choosing an AM system
“When introducing additive methods, foundries should opt for an SLA system that offers a low-cost, high-yield alternative to conventional injection moulding,” explained Andrew Graves, business development manager at Stratasys. “The first step is understanding the size and quality of the patterns and moulds you want to create.
“To achieve this, look at the system’s build area and whether it can produce large parts at production volumes,” added Graves. “Ultimately, the technology must deliver highly accurate patterns and moulds that will provide confidence in the resulting metal part.”
Important factors in system choice include size and post-processing requirements. For example, the Stratasys NEO800 SL system has a large build area of 800 x 800 x 600 mm, making it suitable for supporting larger castings. Becase it can print parts with outstanding surface finishes, any patterns or moulds require little sanding — keeping post-processing to a minimum. Usually, the only areas that need sanding are those where support structures have been removed.
It’s safe to say that metal casting has come a long way from its fourth-millennium BC roots. Today, this process can benefit from digital techniques, such as 3D printing, which can augment both sand and investment casting. While this technology offers so much potential, it’s important that foundries carefully select AM systems and material that will help them meet customer needs, to ensure they get the best return on investment.For more information on 3D printing systems and materials for metal casting applications, visit https://www.tritech3d.co.uk/contact/.
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